Planar magnetics is an advantageous alternative to the conventional iron cores, when a low profile is desired for inductive components. This alternative has been used generally, e.g. when implementing transformers on a circuit board.
In these so-called planar transformers, the transformer coils are integrated on the printed board. Such a structure is illustrated in FIG. 1, which shows four conductive layers A to D of a multiple layer circuit board PCB, whereby the desired winding pattern WP1 . . . WP4 is implemented for each layer. Together, these winding patterns form the transformer windings (primary and secondary windings). The transformer core in the case shown as an example in the figure is formed by E-profiled ferrite pieces F1 and F2, for which the circuit board has corresponding openings H1 . . . H3 extending through the board at the arms of the E. The ferrite pieces are pushed into the openings from opposite sides of the circuit board, whereby they will be placed opposite to one another at the openings so that the end surfaces of the E's arms are against each other.
Conventionally, the ferrite couple has been secured to the circuit board by gluing, so that one ferrite piece is first glued to the circuit board and then the ferrite pieces are glued together. However, gluing is a work step generally perceived as a health hazard, even despite the fact that efficient suction fans are used in the working space. Another drawback of gluing is that, once glued together, it is impossible later to remove the ferrite pieces without damaging the circuit board.
Another known method of securing ferrite pieces is to lock them to one another with the aid of a suitable mechanical locking body. Such a locking method is illustrated in FIG. 2, which is a cross-sectional view of ferrite pieces secured to one another and to a circuit board. The mechanical locking body CL1 attaching the ferrite pieces to each other is formed by a spring-like metal strip, which is pushed through the openings (H1, H3) in the circuit board so that it is located around the ferrite pieces. The metal strip functions as a spring pressing the ferrite pieces against each other (this pressing function of the spring is indicated by arrows P in the figure). In FIG. 2 only the upper ferrite piece F1 is E-profiled, whereas the lower piece F2 is slab-like.
By using only a mechanical locking body the drawbacks caused by gluing are avoided. However, attaching by using only a mechanical attaching body has the drawback that in certain operating environments the ferrite pieces can rub against the circuit board. This may have the result that the protective coating of the circuit board wears off and the ferrite piece contacts and damages the conductive pattern on the circuit board. Such an operating environment where vibration occurs can be e.g. in means of transportation, e.g. in trains or street-cars.